The present invention relates to a process for the production of isocyanates in the gas phase in which by-products, such as ammonium chloride, are selectively separated from the gas phase by desublimation.
In the preparation of isocyanates by phosgenation of the corresponding amines in the gas phase, impurities in the starting materials or cleavage reactions on heating and overheating of the amines during the transformation into the gas phase result in the formation of ammonia, which can react with hydrogen chloride liberated in the isocyanate formation to form ammonium chloride.
Particularly in the gas treatment sections of the production facility, this leads to solid deposits and pressure drops, so that the plant has to be regularly shut down and cleaned.
This fundamental problem in the preparation of isocyanates by gas-phase phosgenation is disclosed in the prior art but solutions for eliminating it have not yet been proposed.
In principle, however, measures for particle separation from fumes or waste air streams are known. On the basis of the prevailing physical separation principle, customary dust separators are divided into four basic types, with mixed forms of these basic types also being used (e.g., wet electrofilters).
In inertial separators (cyclone, settling chamber and vortex chamber), the mass forces, gravitation force, centrifugal force and inertial force are utilized. They are used as a rule for product recycling or as a preliminary separator. The principle of the wet separator (Venturi scrubber, rotary scrubber, jet scrubber) is based on an accumulation of the dust particles on dispersed liquid drops, the diameter of which is at least an order of magnitude greater than that of the dust particles. Depending on the application, scrubbers may be effective simultaneously as dust separator, quencher, humidifier and/or absorber (e.g., crude gas pretreatment for solvent separation, waste incineration plants). A disadvantage of these separators is the need for frequent aftertreatment of the resulting wastewaters. Electrofilters are the most frequently used plants for processes with a high waste gas temperature and high waste gas volume flow rates such as those which occur in large furnaces, cement furnaces and iron ore sintering furnaces. The dust particles acquire a charge in an electrical field and migrate to the collecting electrode, where they are deposited. Filtering separators utilize the filter effect of woven fabrics, felts, mineral fibers or stainless steel fibers.
A disadvantage, particularly with the separators which operate according to the principle of a gas scrubber, is the comparatively high pressure drop.